Thermal actuators are devices that convert thermal energy into mechanical energy by sensing changes in temperature and reacting to these changes by exerting a force or bending moment. This invention relates to the cooling of certain materials termed shape memory effect (SME) materials which are able to remember a previous shape and thereby operate as thermal actuators to generate force or motion.
"Shape memory effect" (SME) is a term that describes the ability of some materials which when deformed relatively easily below a first temperature are able to recover their original shape, while exerting substantial stress, when subsequently heated above a second higher temperature. This property is associated with the appearance and disappearance of a particular crystal structure within the material termed martensite. Martensite is generated by cooling the shape memory effect material below the martensitic finish temperature M.sub.f where it is all martensite. The shape memory effect material is all austenite above the austenitic finish temperature A.sub.f. When deformed while below the martensitic finish temperature and then heated to above the austenitic temperature, the alloy returns to its shape existing before the deformation. This invention is concerned with rapidly cooling actuators fabricated from shape memory effect materials during the cooling portion of the thermal/mechanical cycle, that is, when the actuator is returned from the austenitic state to the martensitic state.
In summary the shape memory effect means that after deforming the shape memory effect material below a first temperature M.sub.f where its structure is martensite, the shape memory material will exert substantial force in recovering, fully or partially, its original shape on subsequent heating over a second higher temperature.
Many metals, plastics and ceramics are known to exhibit the shape memory effect. The extensive list of metal alloys include the copper alloy systems of Cu--Zn, Cu--Al, Cu--Au, Cu--Sn and the many ternary alloys formed from these binary alloy systems by adding a third element. Among others, the alloys of Au--Cd, Ni--Al, Fe--Pt, Ti--Pd, In--Tl, Fe--Pd, Mn--Cu and their ternary alloys, exhibit the shape memory effect. A shape memory effect alloy with exceptional properties is a nickel-titanium alloy known as Nitinol, which is based on equiatomic weights of Ti and Ni. There is also an extensive list of ternary alloys based upon adding a third element to the nickel-titanium binary system. Ni--Ti alloys are able to completely recover from two percent to as high as ten percent of plastic strain, depending on desired fatigue life, by heating above the austenitic finish temperature A.sub.f while yielding forces and/or moments based upon the creation of internal stresses on the order of 100,000 psi.
The use of thermal actuators offers several advantages including high force levels, large movements, small size, multiple actuation modes (linear, bending, torsion and combined), high work performance per unit volume and per unit weight and motion completed in a narrow temperature range. This invention anticipates all of these advantages from an internally cooled shape memory effect material micro-actuator.
The shape memory effect alloys cited above and the multiplicity of additional materials are per se known for their shape memory effect and are included under the scope and spirit of this invention.
The shape memory effect has been applied to a wide variety of applications, many of which make use of its shape recovery mechanism for actuation purposes. Actuators can take many configurations, including but not limited to tensile, cantilever, torsional, helical or combinations thereof.
As an example of SME material actuator operation, typical tensile and cantilever actuators employ a SME material member and a return bias spring mechanically connected in some manner to the SME material device. When such a SME material device is heated, by Joule heating or some other means, the SME material member contracts to perform a work function. The bias spring returns the actuator to its original position after cooling. This invention anticipates this by incorporating the bias spring as an integral part of the microactuator as the resilient component of a composite material in mechanical communication with the shape memory material component.
A second type of actuator, called a differential type, uses two SME material actuators connected in mechanical series whereby heating of one actuator shrinks the combined apparatus in one direction while heating of the second actuator shrinks the apparatus in the opposing direction.
The above embodiments are the uni-directional mode of operation of the SME material actuator wherein the shape memory effect appears only upon heating. The present invention is not limited to this type but is also envisaged to include the bi-directional type wherein the SME material is trained to operate in one, two or three dimensional directions without aid of biasing members. Such training techniques are known in the shape memory effect materials art.
Joule heating, that is passing an electrical current through the SME material member, is the preferred heating choice because Joule heating is relatively uniform throughout the member cross-section. This type of heating causes a rapid temperature rise and therefore a rapid transformation between the martensitic and austenitic crystal forms. This results in a rapid and accurate actuator response. However, this invention envisages other methods of heating including, but not limited to, heating by radiation, conduction or convection.
Current methods of external cooling of a SME material actuator are much slower because heat from the center of the actuator cross-section must diffuse to the surface. This is a relatively slow process. This process is in series with a second mechanism to remove the heat from the surface to the surroundings which has a small heat transfer coefficient at the low temperatures at which SME materials operate. The rate of cooling is limited by the ratio of surface area, as measured by the perimeter of the cross-section, to the volume, as measured by the area of the cross-section of the actuator. This ratio of surface to volume decreases inversely as the diameter of the actuator increases, meaning that the larger the actuator diameter the slower it cools and therefore the slower is the actuator's response. This is exacerbated by the martensitic transformation being diffusion less and progressing so fast that it is essentially adiabatic. Thus the heat of transformation adds to the heat load that must be removed from the center of the actuator through the surface. All of these and other factors combine to limit the rate of cooling of shape memory effect actuators by current cooling methods and therefore lengthen their response times. This is the major reason why useful shape memory effect material actuators are currently limited to small diameters.
Considering all of the above limitations to the current state of the art, it is therefore desirable to cool shape memory effect (SME) material micro-actuators internally using a mechanism capable of transporting large heat fluxes per unit area. This invention relates to using micro-heat pipes as a means of heat removal from the interior of SME material actuators.
Heat pipes are devices that are able to carry very large heat fluxes by utilizing the heat of vaporization of a fluid. A specified amount of fluid is sealed within an elongated enclosure with the heat flux entering through the enclosure wall at one end, termed the evaporator, causing evaporation of the fluid. The resulting vapor phase travels the length of the enclosure via a central vapor space, and condenses on the cooler opposite end of the enclosure where the heat of condensation is removed to the outside through the enclosure wall. The liquid phase is returned to the evaporator end by capillary action via either arteries contained within the enclosure walls or via fine mesh wick structure lining the walls of the enclosure and may be any shape or geometry. Alternatively, the liquid phase can be returned by gravity flow. Gravity flow types of micro-heat pipes are also termed thermosyphons. This invention relates to embedding one or multiple micro-heat pipes in the interior cross-section of shape memory effect material actuators to achieve short actuator response times. A micro-heat pipe may have multiple evaporator and condenser regions whereby adjacent regions of opposite type are in thermal and fluidic communication.
In Cotter's original proposal, "micro" is defined in the art to be a heat pipe small enough that the mean curvature of the vapor-liquid interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel. In practice, this translates into an enclosure with an internal vapor space channel with an approximate diameter of 100 to 1000 micro-meters.
The temporal response of micro-heat pipes is very fast relative to other forms of cooling. However, it is not as fast as Joule heating. This invention envisages a temporal relationship between Joule heating and micro-heat pipe cooling which serves to minimize heat loss during the heating portion of the SME material actuator thermal cycle. During the heating portion of the thermal cycle the onset of micro-heat pipe operation is delayed which accommodates the time needed to complete the heating portion of the SME material actuator cycle. Once started, after an initial delay, the rate of micro-heat pipe cooling is extremely rapid. Other forms of cooling, for example convective cooling, cause inefficiencies due to the removal of heat by the cooling means during the heating portion of the thermal cycle. The present invention avoids this and thereby avoids expensive means of alternatively turning off the cooling means during the heating portion of the thermal cycle. This is important, for example, in robotics applications where precise locational control is important. Uncertainty in heating or cooling results in uncertainty in robotic location.
Because shape memory effect material actuators currently must be cooled through their exterior surface, the environments into which they may be placed are limited to relatively low temperatures and have relatively high thermal conductivities. They cannot, for instance, be embedded into insulative materials such as advanced composites without suffering severe time response degradation. This is due to the extra time required for heat to dissipate from the actuators during the cooling portion of the shape memory effect material thermal cycle. See U.S. Pat. No. 5,114,104 to Cincotta et al for such an application to control surfaces in which cooling is inadequate. This invention solves these problems by providing for internal cooling wherein the operation of the SME material actuator becomes indifferent to the environment into which it is placed. Thus this invention relates to rapidly actuated members constructed from composite materials comprising a low thermal conductivity matrix.
Internally cooled SME material micro-actuators embedded into a second material in one, two or three dimensional arrays can serve to change the external shape of the composite body. The second material can be made from an electrically insulative material so that Joule heating may be used to heat the SME micro-actuators. The SME micro-actuators can be trained to assume one shape on heating and another on cooling as is known in the art. Alternatively, a resilient member can be employed to return the composite material to a second shape upon cooling when the shape memory effect material is in its martensitic state. Such a resilient member can be made a part of the SME material micro-actuator or can be the second material into which the SME material micro-actuators are embedded as is envisaged in this invention. Such one, two and three dimensional SME micro-actuator composite structures would be useful for fluid control surfaces, adaptive optic surfaces and robotics.
Currently shape memory actuators cannot operate in an environment with a temperature higher than the austenitic finish temperature A.sub.f, and don't operate well in environmental temperatures above the martensitic finish temperature M.sub.f. Shape memory effect materials have relatively low transformation temperatures, Ni--Ti and Cu--Zn--Al, for example having transformation temperatures between -200.degree.0 C. and +120.degree. C., this severely limits the application of actuators fabricated from currently available shape memory effect materials to low temperature environments. Because the present invention is internally cooled, these limitations can be eliminated by placing a second heat-resisting material around the actuator and cooling it internally. Alternatively, a plurality of SME material micro-actuators containing one or multiple micro-heat pipes can be embedded into a matrix material with the composite structure able to be actuated in one, two or three dimensions while exposed to a high temperature environment.
Additionally, the thermal gradients between the interior and surface of SME material actuators resulting from surface cooling methods leads to increased hysteresis and to the degradation of the SME material's thermal stress fatigue life. Hysteresis is manifest in the temperature difference measured between the temperature at which the transformation of the shape memory material (SME) from austenite to martensite starts upon cooling, termed the M.sub.s temperature, and the temperature at which the reverse transformation from martensite to austenite starts, termed the A.sub.s temperature, on subsequent heating. A large hysteresic temperature difference limits the accuracy of an actuator. A large temperature gradient across the shape memory actuator cross-section leads to thermal stress fatigue and failure. Placing the cooling means in the interior, as does this invention, reduces this temperature gradient and improves the fatigue and hysteresis characteristics.